240 FLUID MOTIONS but it remains rather an inversion of ordinary ideas that we should havi deduce the forces from the motion, rather than the motion from the force The application of the principle is not always quite straightforw Consider a tube of slightly conical form, open at both ends, and sup] that we direct upon the narrower end a jet of air from a tube having same (narrower) section (fig. 5). We might expect this 'jet to enter Fig. 5. conical tube without much complication. But if we examine more cloi a difficulty arises. The stream in the conical tube would have diffe: velocities at'the two ends, and therefore different pressures. The pressi at the ends could not both be atmospheric. Since at any rate the press at the wider delivery end must be very nearly atmospheric, that at narrower, end must be decidedly below that standard. The course of events at the inlet is not so simple as supposed, and the apparent con diction is evaded by an inflow of air from outside, in addition to the which assumes at entry a narrower section. If the space surrounding the free jet is enclosed (fig. 6), suction is tl' developed and ultimately when the motion has become steady the jet en the conical tube without contraction. A model shows the effect, and principle is employed in a well-known laboratory instrument arranged working off the water-mains. I I Fig. 6. I have hitherto dealt with air rather than water, not only because makes no mess, but also because it is easier to ignore gravitation. ] there is another and more difficult question. You will have noticed thai our expanding tubes the section changes only gradually. What happ when the expansion is more sudden—in the extreme case when the diam« of a. previously.uniform tube suddenly becomes infinite? (fig, 3) -withw is necessarily increased. And when we enquire how the additional velocity in passing from a wider to a narrower place is to be acquired, we are compelled to recognize that it can only be in consequence of a fall of pressure. The suction at the narrows is the only result consistent with the great principle of conservation of energy;ory can give a satisfactory account.length.